Current methods of determining whether a vehicle is compliant with emission standards include open path and closed path emissions measurement systems. In a closed path system, an emission sensor is directly connected to the exhaust of the vehicle, such as by insertion into a tailpipe. An open path vehicular emissions measurement system collects data by a means other than a direct connection to the tailpipe, such as a remote sensor that analyzes the individual components of emissions. Open path vehicle emission systems are often preferable to closed path systems because they can be used in numerous locations and do not require the vehicle to stop for testing.
Various open path emission sensing systems have been known. One such device uses a radiation source on one side of a roadway that projects a beam across the roadway to be received by a detector. The radiation source and the detector are located on opposite sides of the roadway. The radiation source emits light spectra that may be used to detect an emission signature by way of absorption of light, or which alternatively may be used to excite emission components so as to cause the components to emit light. The detected emission signature can then be used in various applications, such as the measurement of a vehicle's compliance with emission limits and the determination of the type of fuel that a vehicle is using.
A disadvantage of many known arrangements is that the radiation sources and detectors must be placed on opposite sides of the roadway from each other. Since both the detectors and radiation sources require power to operate, this means that a separate power supply must be provided on each side of the roadway.
Furthermore, current open path embodiments are unable to maintain stability of measurements throughout the diurnal pattern of daytime-nighttime temperatures. Part of the reason for instability is a lack of effective thermal control of the detecting components of the emissions measurement system. Frequent recalibrations of the instrumentation are required, due to a baseline shift (zero drift) of the measuring system, caused at least in part by thermal instability of the detecting components of the system. For many systems, an increase in detector temperature can result in lowered sensitivity to light, which is seen in data as a rising baseline of measurement, and therefore causing data to move in the negative direction (negative bias). The opposite is true for falling temperatures.
Additionally, open path instrumentation in particular is susceptible to increased noise for each measurement, with increasing detector temperature. This is a problem especially for measurements of very small concentrations of gases of interest, where the concentration of gas may be within the noise of the measuring instrumentation. Ideally, detectors are chilled to close to absolute zero, however this is not practical or safe for portable instrumentation.
Some systems rely on metal to air heat transfer for cooling. The problem with this approach to cooling is that ambient air can be tainted with dust and other contaminants that depose onto the optical components of the measurement system, reducing the effectiveness of the system in determining a concentration of a gas or particles of interest, and requiring more frequent periodic maintenance.
Accordingly, it is desirable to provide an improved optical transmission, reflection, and detection system that can measure particulate matter and gaseous emissions measurements, along with an improved correlation opacity measurement as herein disclosed.